Recently, information recording mediums usable for recording, erasure, rewriting and reproduction of information by laser light radiation have been widely researched and developed, and made into commercial products. Such an information recording medium has an information layer including a recording layer formed of a thin film of a phase-changeable recording material or the like. For recording information, the recording layer is irradiated with laser light and thus locally heated to form areas having different optical constants because of different radiation conditions. For example, information is recorded by changing the state of the phase-changeable material of the recording layer between a crystalline phase and an amorphous phase by heat generated by laser light radiation. Specifically, for example, a combination of a space and a mark represented by lengths based on a signal modulated by information to be recorded is formed on a track of the recording layer.
Among information recording mediums using a phase-changeable material for the recording layer, a rewritable information recording medium allows information to be erased or rewritten because the phase-changeable material is reversibly phase-changeable. In the case of such a rewritable information recording medium, the initial state of the recording layer is generally the crystalline phase. For recording information, laser light of a high power is directed toward the recording layer to melt a part of the recording layer. Then, the recording layer is rapidly cooled to put the part of the recording layer irradiated with the laser light into the amorphous phase. For erasing information, laser light of a power lower than that used for recording is directed toward the recording layer to raise the temperature of a part of the recording layer. Then, the recording layer is slowly cooled to put the part of the recording layer irradiated with the laser light into the crystalline phase. In addition, laser light of a power modulated between a high power and a low power may be directed toward the recording layer. Thus, while the recorded information is erased, new information can be recorded. Namely, rewriting is possible.
In order to carry out erasure or rewriting of information at high speed, it is necessary to change the phase from the amorphous phase to the crystalline phase within a short time. Namely, in order to realize high erasure performance in a rewritable information recording medium, it is necessary to use a phase-changeable material having a high crystallization rate for the recording layer. In the case of a write once information recording medium using a material which is not reversibly phase-changed for a recording layer, information rewriting is impossible. On such an information recording medium, information can be recorded only once.
Information recorded on an information recording medium is reproduced by detecting a light amount change of the reflected light based on the reflectance difference between the crystalline phase and the amorphous phase. Specifically, this is performed by irradiating the information recording medium with laser light which is set to a certain reproduction power and detecting the intensity of the light reflected from the information recording medium as a signal. The intensity of the reflected light is in proportion to a product of the reflectance of the information recording medium and the reproduction power of the laser light. In general, as the intensity of the reflected light is higher, the information reproduction signal quality is higher. Therefore, it is preferable that the reproduction power is higher.
However, where the reproduction power is too high when information is to be reproduced from a recordable information recording medium such as a rewritable information recording medium or a write once information recording medium, the state of the mark or space formed on the recording layer of the information recording medium is changed, and the quality of the reproduction signal to be obtained is deteriorated. Namely, the information represented by the mark or space formed on the information recording medium is deteriorated. For this reason, the energy of laser light for irradiating the information recording medium for reproduction, namely, the reproduction power, is set not to be too high (see Patent Document No. 1). The degree at which information deterioration due to reproduction is unlikely to occur is called reproduction durability. Hereinafter, the upper limit of the reproduction power at which the information remains non-deteriorated when being reproduced is called “upper limit reproduction power”. As the reproduction durability of an information recording medium is higher, the upper limit reproduction power of the information recording medium is higher. The upper limit reproduction power is determined for each information layer, and is one of the characteristics of each information layer.
Recently, various technologies have been studied in order to increase the capacity of information recording mediums. For example, according to one reported technology, a rewritable information recording medium including two information layers is used. By the laser light incident on one surface of the recording medium, information recording on, or information reproduction from, the two information layers is carried out (see Patent Document No. 2; Patent Document No. 3). With this technology, the recording capacity of the information recording medium can be doubled by using two information layers.
In an information recording medium allowing information to be recorded on, or reproduced from, two information layers by laser light incident on one surface thereof, information recording on, and reproduction from, an information layer farther from the incidence side (hereinafter, referred to as the “first information layer”) is carried out by the laser light which has been transmitted through an information layer closer to the incidence side (hereinafter, referred to as the “second information layer”). Where the transmittance of the second information layer is low, the energy of the laser light reaching the first information layer is attenuated. In addition, since the light reflected by the first information layer is transmitted through the second information layer again, the light reflected by the first information layer is further attenuated and the intensity thereof is decreased. Therefore, the quality of the reproduction signal based on the reflected light is lowered.
Considering the light attenuation in the second information layer, it is conceivable to record the information on the first information layer with more intensive laser light. However, in this case, the first information layer needs to be irradiated with laser light of a higher power. When the power of the laser light exceeds the limit of the recording apparatus, preferable recording cannot be realized, and the recording quality is deteriorated. In this case, the second information layer is irradiated with more intense laser light with no deterioration. Therefore, in the information recording layer closest to the laser light incidence side, signal deterioration due to reproduction is likely to occur, and it tends to be difficult to improve the reproduction durability.
For the above-described reasons, it is preferable that the second information layer has a maximum possible transmittance. In order to realize an information recording medium having an increased number of information layers, for example, three or four information layers, for the purpose of increasing the capacity, it is necessary to further increase the transmittance of the information layer on the incidence side (third or fourth information layer). Especially, it is desirable that an information layer closest to the laser light incidence side has a maximum possible transmittance because light is transmitted through this information layer in order to record information on, or reproduce information from, an information layer farther from the laser incidence side. In general, recording materials have a large extinction coefficient. Therefore, it is preferable that the recording layer of the information layer on the laser light incidence side is thin in order to have a high transmittance.
This tendency regarding the reproduction durability is applicable both to a rewritable information recording medium and a write once recording medium. In the case of a rewritable information recording medium, erasure performance of the recording layer also needs to be considered as described above. Therefore, it is necessary to adjust the recording layer or the like so as to provide both of good erasure performance and good reproduction durability.
Citation List
Patent Literature
                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-14679        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2001-36130        Patent Document No. 3: the pamphlet of International Publication 03/025922        